Recently a simple matrix LCD has been widely used for electronic calculators, electric home appliances such as radios and measuring equipment, etc. For application to these devices, it is desirable for the LCD to have less power consumption, less driving voltage, good contrast, less crosstalk, viz., less phenomenon of half-selected segments appearing like those selected because of potentials applied there, and yet less expensive.
A conventional simple matrix driving system for displaying a liquid crystal panel has been a multiplex system, viz., a system of a line sequential AC drive. The system has common electrodes and segment electrodes. The common voltage waveforms are applied to the common electrodes each in a manner of time division line sequences. The signal voltages are each applied to the segment electrodes. Then the selected points are displayed by the combination of these two types of the voltages. The system is widely adopted because less signal lines are needed for driving.
As is well known, electrolysis occurs when a direct current is continuously applied to the liquid crystal. Therefore, the mean value of the electric field applied to the liquid crystal during a certain period needs to be zero in order to prevent the electrolysis.
For obtaining a good display quality, the system described above adopts the system of applying a bias voltage for the proper setting of the effective values "Von" and "Voff", which are applied to the selected points (an active portion of the liquid crystal) and to the half-selected points (an inactive portion of the liquid crystal) respectively. The system needs three or more values of voltages, viz., voltages of a power source voltage level, zero potential and one or more values of an intermediate level. A popular example is 1/2 duty-1/2 bias or 1/3 duty-1/3 bias driving system.
On one hand it is necessary to apply the "Voff" voltages to the half-selected segments for the speed up of the response of the liquid crystal; on the other hand, the larger "Von/Voff" ratio is, the better the contrast is.
The following is an explanation of the example of 1/2 duty-1/2 bias or 1/3 duty-1/3 bias driving;
FIG. 4 shows the structural diagram of the liquid crystal display portion of 1/2 duty-1/2 bias with seven segments forming a numeric "8". The two common electrodes C1 and C2 are commonly coupled with each of the segments, and the four segment electrodes S1 through S4 are commonly coupled with each of the segments. The shaded segments in FIG. 4 are under driving.
FIG. 6 shows the common voltage waveforms of C1 and C2 of the conventional liquid crystal driving circuit 1 of FIG. 8(a). FIG. 6 also shows the segment voltage waveforms of S1 and S2 of the same circuit, and the voltage waveforms of the potential differences between the common electrode C1 and the segment electrodes S1 and S2. This 1/2 duty-1/2 bias common voltage waveforms have the three voltage levels of VDD, V1 and V2, and then, the segment voltage waveforms have the two voltage levels of VDD and V2. The liquid crystal driving circuit 1 gets these voltages from the voltage dividing circuit 2. The voltage dividing circuit 2, having voltage-dividing resistors shown in FIG. 9(a), generates the voltage levels of V1 and V2 by dividing the power source voltage VDD which comes from the power source 3. With a variable resistor Rv in FIG. 9(a), the potential levels between VDD and V2 are adjusted for the control of the display intensity.
From the voltage waveforms of FIG. 6, it is understood that, for example, the voltage of effective value V1((1.sup.2 +2.sup.2)/.sup.1/2 is applied between the common electrode C1 and the segment electrode S1. Then, the segment 11 between the common electrode C1 and the segment electrode S1 is driven because the voltage is higher than the threshold voltage for ON of the liquid crystal. Between the common electrode C1 and the segment electrode S2, the voltage of the effective value V1(1.sup.2 /2).sup.1/2 is applied. However, the segment 12 between the common electrode C1 and the segment electrode S2 is not driven because the voltage is lower than the threshold voltage for ON of the liquid crystal.
FIG. 5 shows structural diagrams of the liquid crystal display portion of 1/3 duty-1/3 bias with seven segments forming a numeric "8". The system has the common electrodes C1 through C3 which are commonly coupled with each of the segments, and the segment electrodes S1 through S3 which are commonly coupled with each of the segments. The shaded segments are under driving.
FIG. 7 shows the common voltage waveforms of the common electrodes C1 through C3 of the conventional liquid crystal driving circuit 4 of FIG. 8(b). FIG. 7 also shows the segment voltage waveforms of the segment electrodes S1 through S3 of the same circuit, and the voltage waveforms of the potential differences between the common electrodes C1, C2 and the segment electrodes S1, S3. These 1/3 duty-1/3 bias common voltage waveforms and the segment voltage waveforms have four voltage levels of VDD, V1, V2 and V3. The liquid crystal driving circuit 4 of FIG. 8(b)gets these voltages from a voltage dividing circuit 5. The voltage dividing circuit 5, having voltage-dividing resistors of FIG. 9(b), generates the voltage V1, V2 and V3 by dividing the power source voltage VDD from a power source 9. With a variable resistor Rv in FIG. 9(b), the potential levels between VDD and V3 are adjusted for control of a display intensity.
From the voltage waveforms of FIG. 7, for example, the voltage of the effective value V1((1.sup.2 +1.sup.2 +1.sup.2)/3).sup.1/2 is applied between the common electrode C1 and the segment electrode S1. However, the segment 21 of FIG. 5 between the common electrode C1 and the segment electrode S1 is not driven because the effective value is lower than the threshold voltage for ON of the liquid crystal. Between the common electrode C2 and the segment electrode S3, the voltage of the effective value V1((1.sup.2 +3.sup.2 +1.sup.2)/3).sup.1/2 is applied. Then, the segment 22 between the common electrode C2 and the segment electrode S3 is driven because the effective value is higher than the threshold voltage for ON of the liquid crystal.
As described above, the conventional driving system needs the control of three or more voltages. However, the digital circuits of microcomputer, gate array, etc. are operated on the binary basis of on-off. Therefore, it is practically difficult to adopt the direct control system for the digital circuits like microcomputer, gate array, etc., because a complicated structure is needed for the direct control of three or more voltages on the circuits.
The driving system described above receives a plurality of voltages, in some cases, from the divided voltages which are generated by dividing the power source voltage with the voltage dividing resistors. In these cases, the output impedance of the power source to the LCD depends on the voltage dividing resistors. Then, if the resistance values of the dividing resistors are increased for a purpose of low power consumption, the driving voltage waveforms are distorted by the resistance load and the capacitance of the liquid crystals. Since the capacitance is different by each segment, the display intensity of the selected segments differs by each segment. Then, the uniform contrast is not obtainable and also an uneven crosstalk occurs on the half-selected segments.
The digital circuits have come to be driven with lower and lower voltages and the microcomputers driven with less than two volts are now in use. However, the driving voltages are too low for the conventional liquid crystal driving system described above, so that the liquid crystal cannot be driven in a visible range without using a voltage boosting circuit.
In case of the conventional driving system described above, the display intensity is adjusted by changing the driving voltages using the variable resistor for instance. However, it is difficult to adjust the driving voltage values directly on the digital circuits.
Then, a binary voltage single power source multiplex driving system is proposed. And there is another proposal of binary voltage driving for obtaining a uniform contrast, that is, a frame period is divided into some timing periods and the contrast is adjusted at one of the timing periods.
However, an easier and more flexible contrast and display intensity adjustment system is desired.
On an LCD equipped relatively high voltage operated appliance, when binary voltage driving is made using the same power source, it is necessary to step down the voltages applied to the LCD by dividing the power source voltage using the variable resistor, for instance. Then, additional component parts are needed. Especially when the LCD is driven with the voltages divided by the variable resistor, like in the case of the bias driving system as described above, and the resistance value of the voltage dividing resistor is increased for the purpose of low power consumption, the driving waveforms are distorted by the resistance load and the capacitance of the segments. Then, the display intensity of the segments to be displayed differs by each segment. In such case, even if the contrast adjustment is made by the method described above, the uniform contrast is not obtainable, and in addition to that, the voltages applied to the half-selected segments becomes uneven and an uneven crosstalk occurs. Therefore, a better driving system is desired, with which the effective values of the voltages applied to the LCD are adjustable without occurrence of the above problems.